1. Field of the Invention
The present invention relates to optical disks, and more specifically, to a digital signal modulation method used in an optical disk encoding system.
2. Description of the Prior Art
Due to having outstandingly high capacity and being very compact in size, optical disks such as CDs and DVDs have become popular media choices for data storage. Accordingly, the use of CD and DVD drives to access this data have increased.
The following description explains how a CD drive accesses data on a CD. When burning an 8-bit data byte to the CD, the CD drive controls its pickup head to emit a laser beam onto spiral grooves on the CD according to the data byte. The laser beam volatilizes chemicals in the grooves in order to form a plurality of pits and lands corresponding to the data byte. When reading the data on the CD, the CD drive again controls its pickup head to emit a laser beam onto the CD. Since the resulting light reflected by the pits and the lands in the grooves differ from each other (the pits reflect the beam, while the lands scatter it), the pickup head reads the data from the CD according to these differences in reflection. (The reflection by the pits is more intense than that by the lands).
In order to ensure normal tracking of the pickup head in the CD drive, the data byte is required to be encoded in a specific manner before being burnt onto the CD. Such kind of data encoding includes an eight-to-fourteen modulation (EFM) process and a merging channel bit attaching process (in case of a CD). The EFM process converts the (8-bit) data byte to a 14-bit channel bit code according to a contrast table. The merging channel bit attaching process involves choosing one of four sets of 3-bit merging channel bits (000, 001, 010, 100) according to the variation of a digital sum value (DSV) corresponding to the 14-bit channel bit code and every bit (3T-11T) of another 14-bit channel bit code next to the 14-bit channel bit code. In this way, the chosen set is attached to the 14-bit channel bit code to form a 17-bit channel bit code. Thus the CD drive controls the pickup head to emit the laser beam onto the CD according to the 17-bit channel bit code and forms pits and lands corresponding to the 17-bit channel bit code in the grooves.
The following description explains the calculation of the DSV corresponding to a channel bit code. If the channel bit code is 10010000000010010, the corresponding DSV is 1232 10(xe2x88x921)(xe2x88x922)(xe2x88x923)(xe2x88x924)(xe2x88x925)(xe2x88x926) (xe2x88x925)(xe2x88x924)(xe2x88x923)(xe2x88x924)(xe2x88x925), where each xe2x80x9c1xe2x80x9d appearing in the channel bit code corresponds to the points of change (from increasing to decreasing, or from decreasing to increasing) of the DSV.
For instance, assuming there are three data bytes V, I, A to be burned on the CD. The ASCII codes of the letters V, I, A are 01010110b(86d), 01001001b(73d), 01000001b(65d), respectively. The 14-bit channel bit codes corresponding to these three binary codes are 00010010000100,10000001000100,10000100100100, respectively. The 3-bit merging channel bit codes selected according to the DSV of these three 14-bit channel bit codes are 000, 100, 000, respectively. The CD drive controls the pickup head to emit the laser beam according to a burning code being 000100100001000001000000100010010010000100100 100000 (the merging channel bit codes are underlined) in order to form the pits and lands in the grooves of the CD. The DSV corresponding to the burning code is 123210123454321(xe2x88x921)0123456543456787656789(10).
The CD drive controls the pickup head to change the laser emission whenever xe2x80x9c1xe2x80x9d appears in the burning code. It controls the pickup head to stop the laser emission or restart the laser emission in order to form the pits and lands. It can be said that every boundary of a pit and a land corresponds to the appearance of xe2x80x9c1xe2x80x9d. In other words, whenever a xe2x80x9c1xe2x80x9d appears in the burning code, the difference between the accumulated number of pits and that of the lands changes, and the DSV corresponding to each bit of the burning code represents the difference between the accumulated number of pits and that of the lands.
When selecting the 3-bit merging channel bit code to insert between two 14-bit channel bit codes, the number of xe2x80x9c0xe2x80x3xe2x80x9ds between two neighboring xe2x80x9c1xe2x80x3xe2x80x9ds in the 31(14+3+14)-bit channel bit code is required to be between 2 and 10 (3T-11T). The DSV corresponding to the 31-bit channel bit code (i.e. the difference between the number of pits and the number of lands) is also required to be approach 0 in order to ensure the DSV is within a threshold range (i.e. the maximum of the DSV being lower than the upper limit of the threshold range, and the minimum of the DSV being higher than the lower limit of the threshold range). In this way, the power spectrum of a high frequency (HF) signal is with a predetermined range.
The conventional method for selecting the merging channel bit code is to choose one from the four sets of 3-bit merging channel bits (000, 001, 010, 100) according to the variation of the DSV. The one selected is inserted between two neighboring 14-bit channel bit codes (of course limited by a condition of 3T-11T). Briefly, if the DSV corresponding to the previous 14-bit channel bit code approaches 0, choose xe2x80x9c000xe2x80x9d to be the merging channel bit code, otherwise choose one from xe2x80x9c001xe2x80x9d, xe2x80x9c010xe2x80x9d, xe2x80x9c100xe2x80x9d complying with 3T-11T to be the merging channel bit code.
Please refer to FIG. 1 showing the relationship between a conventional burning code and its corresponding DSV. The conventional method for selecting the merging channel bit code only ensures the DSV xe2x80x9clocallyxe2x80x9d approaches 0 (indicated by arrows A, B, and C shown in FIG. 1). However, it is unable to ensure the DSV remains within the threshold range for the long term (indicated by arrow D shown in FIG. 1). If the DSV exceeds the threshold range, the data burned by the CD drive can not be re-read.
It is therefore a primary objective of the present invention to provide a digital signal modulation method for use in a CD/DVD encoding system to ensure the data burned to a CD/DVD can be read correctly by a CD/DVD drive.
Briefly summarized, a digital signal modulation method attaches bits of a predetermined number next to each one of a plurality of first bit streams in order to generate a plurality of second bit streams. The method includes (a) adjusting the bits of the predetermined number attached to a following plurality of first bit streams after generating the plurality of the second bit streams according to the variation of a DSV corresponding to the plurality of bits in the plurality of second bit streams, and (b) attaching the bits of the predetermined number to the first bit stream according to Step (a) and the DSV of each bit in the following plurality of first bit streams after generating the plurality of the second bit streams, in order to make the DSV of the bits of a following plurality of second bit streams after generating the plurality of the second bit streams approach 0.
These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.